1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device and, more particularly, to an active matrix liquid crystal display device which can provide a bright display.
2. Description of the Related Art
A TN type active matrix liquid crystal display device comprises: a first substrate on which a plurality of pixel electrodes and active elements respectively connected thereto are arranged in the form of a matrix; a second substrate having a counter electrode opposing the pixel electrodes; a frame-like seal member for joining the first and second substrates; a nematic liquid crystal sealed in a space defined by the first and second substrates and the seal member; and polarizing plates arranged to sandwich the first and second substrates.
Aligning films for restricting the aligning direction of liquid crystal molecules are formed on the surfaces of the first and second substrates on which the electrodes are formed. The liquid crystal molecules are twisted/aligned between the first and second substrates at a twist angle of almost 90.degree.. The pair of polarizing plates are arranged such that their transmission axes are almost parallel to each other.
In the TN type active matrix liquid crystal display device, external incident light is linearly polarized by one of the polarizing plates and incident on the liquid crystal layer. Of the light passing through the liquid crystal layer, only light components parallel to the transmission axis of the other polarizing plate emerge therefrom. For this reason, the light amount loss owing to light absorption in the polarizing plates is large, resulting in a dark screen.
This problem is especially conspicuous in a reflection liquid crystal display device having a reflecting plate arranged on the rear side of the device. More specifically, the reflection liquid crystal display device performs a display operation by using natural light or indoor illumination light. That is, the device is designed to use weak light. In addition, light incident on the upper surface side of the liquid crystal display device passes through each of the pair of polarizing plates twice until the light is reflected by the reflecting plate and emerges from the upper surface side. For this reason, the loss of light is large. As a result, the screen becomes extremely dark.
In a transmission liquid crystal display device used with a backlight arranged on the lower surface side, light incident on the lower surface side passes through each of a pair of polarizing plates once until the light emerges from the upper surface side. For this reason, the loss of light in the transmission liquid crystal display device is smaller than that in the reflection liquid crystal display device. However, the brightness of the screen is 1/2 or less that of illumination light from the backlight.
A conventional color display device colors transmitted light by absorbing light components, of the transmitted light, which have a predetermined wavelength range, thereby displaying a color image. The amount of light absorbed by a color filter is large. Especially in a reflection liquid crystal display device, since light passes through a color filter twice, the display becomes considerably dark.
That is, in a TN type active matrix liquid crystal display device using a color filter, especially the problem of a dark display is posed.
Furthermore, in a conventional active matrix liquid crystal display device, since a capacitor (pixel capacitor) formed by the liquid crystal between each pixel electrode and a corresponding counter electrode is small, a voltage held by each pixel gradually decreases during a non-selection period owing to a leakage current. In order to solve this problem, capacitor electrodes are arranged to oppose the pixel electrodes via a gate insulating film to connect compensation capacitors in parallel to the pixel electrodes. In this method, however, since each capacitor electrode overlaps a corresponding pixel electrode, the opening ratio of each pixel decreases.